JP2003092832A - Controller for self-excited converter for dc transmission - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress the generation of low-order higher harmonics, with a control circuit for a self-excited converter for a DC transmission system. SOLUTION: In a DC transmission system, control circuits 110 and 120 for two self-excited converters 41 and 42 for converting AC into DC or vice versa are constituted of vector control systems for converting three-phase AC into the two-phase DC components of active components and reactive components, and they are connected with each other via a DC circuit. The system is equipped with AC current detection means 111 and 112 which detect the AC of the self-excited converter separately for each phase, a higher harmonic control signal making means 117 which detects higher harmonic components that the user wants to reduce from the detected current separately for each phase and multiplies the detection signal by gain separately for each phase, and an addition means which adds a higher harmonic control signal to higher harmonic signals Va, Vb, and Vc separately for each phase to serve as the original form of the AC output voltage waveform of the self-excited converter separately for each phase. This makes pulses by a pulse generating circuit 119, based on the added signal, and controls the self-excited converter.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a control device for a DC power transmission system including a self-excited converter, and more particularly to a harmonic that flows from the self-excited converter to an AC system or a harmonic that originally remains in the AC system. The present invention relates to a control device having a harmonic suppression function for reducing the noise.

[0002]

2. Description of the Related Art A switching element forming a converter is I
GBT (Insulated Gate Turn-off Bipolar Transisto)
Since a self-excited converter composed of a self-extinguishing element such as r) has the ability to cut off the current flowing by itself,
Active power and reactive power can be controlled independently and at high speed without being affected by the AC system. Therefore, its application to a DC power transmission system is under consideration.

The self-excited converter is composed of a three-phase bridge using a self-extinguishing element, and is generally used with a large number of pulses in order to reduce the low-order generated harmonics of the converter.
However, if the number of pulses is increased, there is a problem that the switching loss of the converter increases and the conversion efficiency deteriorates. On the other hand, when the number of pulses is reduced, the low-order generated harmonics increase,
Since the capacity of the AC filter becomes large, there is a problem that the system cost becomes high.

[0004]

In order to reduce the loss of the converter, the number of pulses of the self-excited converter per unit is decreased, and the number of pulses of the self-excited converter is multiple-connected to increase the equivalent number of pulses. A method is being considered. This is a method in which self-exciting converters having different output voltage phases are connected in a multiple manner to cancel harmonics generated between the self-exciting converters and reduce harmonics in the entire converter station. However, it is necessary to shift the phase shift of the conversion transformer in order to make multiple connections, which complicates the structure of the transformer, increases the cost associated with the increase in the number of self-excited converter units, and complicates control and protection. There is a problem that the reliability of the system decreases.

On the other hand, as a self-exciting converter used for AC motor control or the like, there is known a self-exciting converter that corrects a control signal of the self-exciting converter based on a harmonic component detected from a load current (for example, a special feature). Kaihei 11-225477).
However, in the case of a self-exciting converter for DC power transmission, it is difficult to apply such a single-axis control because high-speed control cannot be performed.

In view of the problems of the prior art, an object of the present invention is to provide a direct current interconnection system which can reduce the AC filter capacity of a conversion station by suppressing low-order harmonics in a self-excited converter. It is to provide a control device for an excitable converter.

[0007]

According to the present invention, a two-axis control self-exciting converter is provided with an active filter function to suppress the generation of low-order harmonics even with the same number of self-exciting converter pulses. It achieves the purpose.

That is, the control device for the self-excited converter comprises a vector control system for converting a three-phase alternating current into a two-phase direct current component of an effective component and an ineffective component, From the detected current, the active filter function of detecting the harmonic component to be reduced for each phase and suppressing the generation of low-order harmonics is provided so as to correct the output voltage command value of the control device. And

More specifically, in a control device of a self-excited converter for DC power transmission, two self-excited converters for converting alternating current to direct current and direct current to alternating current are connected to each other through a direct current circuit. The control device of the excitation converter is composed of a vector control system for converting three-phase alternating current into two-phase direct current components of an active component and an ineffective component, and an alternating current for detecting the alternating current of the self-excited converter for each phase. Detector, harmonic suppression signal creation circuit that detects the harmonic component that you want to reduce from the current detected from each phase for each phase and multiplies the detected signal by gain for each phase, AC output of the self-excited converter Adding means for adding the output signal of the harmonic suppression signal creating circuit for each phase to the output voltage command value for each phase which is the prototype of the voltage waveform, and a pulse is created based on the added signal Pulse generation to control the excitation transducer Characterized in that a circuit.

Further, instead of the above-mentioned AC current detector on at least one side of the self-excited converter, the AC system side current including the AC current and the load current of the self-excited converter is detected for each phase. A second alternating current detector is provided, and the current detected by this is input to the harmonic detecting means of the harmonic suppression signal generating circuit for each phase.

Here, the harmonic detecting means for the harmonic component to be reduced is a method of detecting a harmonic of one order contained in an alternating current by a bandpass filter, and a harmonic of several orders is detected by a bandpass filter. And the method of detecting all harmonic components except the fundamental wave.

According to the present invention, it is possible to suppress harmonics generated from the self-exciting converter for DC power transmission, and further harmonics remaining in the AC system including the load.

[0013]

1 shows an example of the configuration of a self-excited DC interconnection system to which the present invention is applied. Describing according to the reference numerals in the figure, 1 and 2 are alternating current systems, 21 and 22 are impedances representing the impedance behind the alternating current system, 31 and
Reference numeral 32 is a conversion transformer for system interconnection, and 41 and 42 are voltage type self-constituted switching elements having a self-extinguishing function for converting direct current to alternating current or alternating current to direct current. Excitation converters, 51 and 52 are DC capacitors for smoothing DC voltage, and 60 is impedance of DC transmission line. This DC transmission line does not exist in the BTB (Back-To-Back) system of the different frequency interconnection system or the asynchronous interconnection system.

Reference numerals 71 and 72 are AC filters for absorbing high-order harmonics generated from the self-excited converter, 100 is an operation command circuit for instructing the operation of the DC interconnection system, 110 is a control circuit for the self-excited converter 41, 111 is an alternating current detector for detecting the current flowing from the alternating current system to the self-excited converter 41, 1
12 is an AC voltage detector that detects the voltage applied to the self-excited converter 41, and 113 is an effective / invalid output that calculates and outputs active power and reactive power that flow to the self-excited converter 41 from the detected AC current and AC voltage. A power detector, 114 is a DC voltage detector for detecting a DC voltage smoothed by a DC capacitor on the DC output side of the self-exciting converter, and 120 is the self-exciting converter 4.
2 is a control circuit, 121 is an AC system to a self-exciting converter 42
AC current detector that detects the current flowing through the device, 122 is an AC voltage detector that detects the voltage applied to the self-exciting converter 42,
Reference numeral 123 is an active / reactive power detector that calculates and outputs active power that flows into the self-excited converter 42 from the detected AC current and AC voltage, and 124 is a DC capacitor on the DC output side of the self-excited converter. 3 shows a DC voltage detector that detects the generated DC voltage.

FIG. 2 shows a detailed circuit of a control device for a self-excited converter having an active filter function. In general, the control circuit performs power flow inversion, so the forward converter that converts alternating current to direct current and the inverse converter that converts direct current to alternating current are also configured with the same circuit, and operation of forward converter operation or inverse converter operation is performed. The operation mode is switched between the forward converter operation and the inverse converter operation by switching the operation command value according to the state. Therefore, the control circuit 110 of the self-excited converters 41 and 42 of FIG.
And 120 are provided with the same control circuit.

Describing according to the reference numerals of FIG. 2, 115 is an output voltage command value (modulated wave) Va, Vb of the self-excited converter.
An output voltage command value creating circuit for creating Vc, 116 is an adding circuit, 117 is an AC current detector 111 (121) for detecting an AC current flowing in a self-exciting converter, and is included in the AC current for each phase. In the harmonic suppression signal generation circuit including a bandpass filter that detects the harmonic component to be reduced, this output is added to the output voltage command value of the self-excited converter by the addition circuit 116. To be exact, the detected harmonic component is added to the output voltage command value in the opposite phase, that is, subtracted.

Reference numeral 119 denotes P from the output signal of the adder circuit (new voltage command value of the self-excited converter, that is, new modulation wave).
It is a PWM pulse generation circuit that creates a WM pulse. P
The CARR of the WM pulse generation circuit is the carrier frequency, eg a triangular wave signal, that will produce a pulse by comparison with the new modulated wave.

As described above, in this embodiment, since the active filter for reducing the harmonic component is provided on the output side of the output voltage command value generating circuit 115, that is, in the main loop of the control circuit 110 (120), it is self-excited. The harmonics generated by the converter can be surely suppressed.

FIG. 3 shows details of the output voltage command value generating circuit. Since the control device of the self-exciting converter requires high-speed control, it is generally configured by a non-interference vector control system by converting the three-phase AC into the two-phase DC component of the effective component and the ineffective component by dq conversion.

Referring to FIG. 3 in accordance with the reference numerals, the output voltage command value generating circuit 115 includes a three-phase output voltage command value (modulated wave signal) Va, Vb of the self-excited converter from the effective component control circuit and the ineffective component control circuit. Create Vc. Reference numeral 1150a is a first adder circuit for obtaining a deviation between the active power command value Pp and the active power detection value Pf which is one of the outputs of the active / reactive power detection circuit. 1150b is a first operational amplification for operational amplification of this deviation. In the circuit, 1150a and 1150b constitute an active power control circuit that controls the active power of the self-excited converter to a command value.

Reference numeral 1151a is a second addition circuit for obtaining a deviation between the direct current voltage command value Vdp of the direct current transmission system and the detection value Vdf from the direct current voltage detector, and 1151b is a second operational amplifier circuit for performing operational amplification of this deviation. Then 1151a and 11
51b constitutes a DC voltage control circuit for controlling the DC output voltage of the self-excited converter to a constant command value.

Reference numeral 1152 is a signal selection circuit for selecting whether to perform active power control of the self-excited converter or DC voltage control. Generally, on the side of the forward converter that converts AC to DC, the DC voltage of the DC transmission system is changed. The DC voltage control for controlling to the command value is selected, and the active power control circuit for controlling the power supplied to the load to the command value is selected on the inverse converter side for converting DC to AC. Therefore, the DC voltage control circuit and the active power command value are given on the forward converter side so that the DC voltage control circuit is selected by the signal selection circuit 1152, and the active power control circuit is selected on the inverse converter side. Give the command value of.

Although not described, the forward power transmission system can be operated by performing active power control on the forward converter side and DC voltage control on the inverse converter side. Further, as the control system, the active power control circuit can be configured as a limiter of the DC voltage control circuit, and the operation does not change.

Reference numeral 1153a is obtained by converting the output signal of the control circuit selected by the signal selection circuit into the effective current command value Idp and converting the Idp and the three-phase alternating current detected value detected by the alternating current detector into two phases. A third adder circuit for obtaining a deviation from the active current detection value Idf, 1153b is a third operational amplifier circuit for carrying out operational amplification of this deviation, 1153a and 1153b.
And constitute an active current control circuit.

Reference numeral 1154 denotes an active voltage detection value Edf and a reactive current detection value Iqf or a reactive current obtained by converting the output of the active current control circuit and the three-phase AC voltage detection value detected by the AC voltage detector into two phases. In the fourth adder circuit for adding the signal obtained by multiplying the command value by the impedance Xt of the conversion transformer, the effective component control circuit is configured by the circuits 1150a to 1154, and the output of this adder circuit is the effective component voltage command value. Become. Non-interference control with the reactive component control circuit is performed by adding a signal obtained by multiplying the reactive current detection value Iqf or the reactive current command value by the impedance Xt of the conversion transformer.

Next, the invalidity control circuit will be described. 1155
a is a fifth addition circuit for obtaining a deviation between the reactive power command value Qp and the reactive power detection value Qf which is one of the outputs of the active / reactive power detection circuit, and 1155b is a fifth operational amplification for performing operational amplification of this deviation. In the circuit, 1155a and 1155b constitute a reactive power control circuit that controls the reactive power of the self-excited converter to a command value.

Reference numeral 1156a designates the output signal of the reactive power control circuit as the reactive current command value Iqp, and the reactive current detection value obtained by converting the Iqp and the three-phase alternating current detection value detected by the alternating current detector into two phases. A sixth adder circuit 1156b for obtaining a deviation from Iqf is a sixth operational amplifier circuit for carrying out operation amplification of this deviation, and 1156a and 1156b constitute a reactive current control circuit.

Reference numeral 1157 denotes a reactive voltage detection value Eqf and an active current detection value Idf obtained by converting the output of the reactive current control circuit and the three-phase AC voltage detection value detected by the AC voltage detector into two phases, or an active current. A seventh addition circuit that adds a signal obtained by multiplying the command value by the impedance Xt of the converting transformer, and a reactive component control circuit is configured by the circuits 1155a to 1157. The output of this addition circuit is the reactive component voltage command value. Become.

Non-interference control with the active component control circuit is performed by adding a signal obtained by multiplying the active current detection value Idf or the active current command value by the impedance Xt of the conversion transformer. Reference numeral 1158 denotes a three-phase output voltage command value (modulation wave signal) Va, Vb, Vc of the self-excited converter by inverse dq converting the effective voltage command value and the reactive voltage command value and further performing two-phase / three-phase conversion. Is made. The harmonic suppression signal for each phase generated by the harmonic suppression signal generation circuit 117 is added to this signal to generate a new modulated wave, and the PWM pulse generation circuit 119 generates control pulses Pap, Pa for the self-excited converter.
n ... Pcn is created. These pulses are guided to the self-excited converter 41 or 42.

FIG. 4 shows a concrete example of the harmonic suppression signal generation circuit 117. The circuit of FIG. 4 shows one phase, and the same circuit is provided for each phase. Reference numeral 1170 denotes a bandpass filter that receives an AC current detection value for each phase of the AC current detector as an input and detects a harmonic of a desired order to be reduced among the harmonics included in the AC current detection value. A harmonic component included in the detected alternating current is multiplied by a constant by an amplifier provided in the bandpass filter, is guided to the adder circuit 116, and is added to the modulated wave in the opposite phase to create a new modulated wave.

This new modulation wave and carrier signal (triangular wave)
And are compared to create a PWM pulse. In the self-excited converter, this PWM pulse creates an AC output voltage waveform based on a new modulation wave, so that an output voltage waveform in which the harmonics detected by the bandpass filter are suppressed can be obtained in the output voltage waveform. it can. As a matter of course, it is necessary that the harmonic order of the alternating current to be reduced is lower than the order of the frequency of the carrier signal.

By providing the self-exciting converter of the DC transmission system with the active filter function according to the present invention, it is possible to reduce the generation of low-order harmonics even with the same number of pulses of the self-exciting converter. The filter capacity can be reduced, and the cost of the conversion station can be reduced.

FIG. 5 shows another example of the harmonic suppression signal generating circuit. This harmonic suppression signal generation circuit shows one phase when there are some harmonics to be reduced, and the same circuit is provided for each phase. Reference numeral 1174 is a first bandpass filter for detecting one harmonic component of a harmonic to be reduced from an alternating current, 1175 is a second bandpass filter, 11
7n is an n-th band pass filter, and 1176 is an adder circuit for adding the detected harmonic components. This output is guided to the adder circuit 116 of FIG. Is made.

This new modulated wave and carrier signal (triangular wave)
And are compared to create a PWM pulse. In the self-excited converter, this PWM pulse creates an AC output voltage waveform based on a new modulation wave, so that an output voltage waveform in which the harmonics detected by the bandpass filter are suppressed can be obtained in the output voltage waveform. it can.

FIG. 6 shows still another example of the harmonic suppression signal generating circuit. This harmonic suppression signal generation circuit shows one phase for suppressing all harmonics except the fundamental wave,
The same circuit is provided for each phase. Reference numeral 1171 denotes a bandpass filter whose center frequency is the fundamental wave, and 1172 is an adder circuit for subtracting the detection signal of the fundamental wave bandpass filter from the detected value of the alternating current hot water, and the output signal of the addition circuit is an alternating current excluding the fundamental wave. The contained harmonic components are detected.
This signal is gain-multiplied by the constant multiplication circuit 1173 and is guided to the addition circuit 116 in FIG.

The harmonic suppression signal created by the harmonic suppression signal creation circuit 117 is added to the modulation wave in the opposite phase to form a new modulation wave, which is compared with the carrier signal (triangular wave) to generate a PWM pulse. Made In the self-excited converter, this PWM pulse creates an AC output voltage waveform based on a new modulation wave, so that an output voltage waveform in which detected harmonics are suppressed can be obtained in the output voltage waveform. In this case, the harmonic order of the alternating current that can be suppressed is a lower harmonic than the frequency of the carrier signal.

Another embodiment of the present invention is shown in FIG. In this embodiment, not only the harmonics generated from the self-excited converter of the self-excited DC power transmission system are reduced, but also the harmonics remaining in the AC system and generated from nearby loads are suppressed by the self-excited converter. You can Here, an example in which the present invention is applied to the side of the self-excited converter 42 is shown in order to explain suppression of harmonics generated from the load.
Of course, if the other end also has the same configuration, the harmonics remaining in the AC system can be suppressed.

In FIG. 7, the same reference numerals as those used so far represent the same functions, and therefore a different new one will be described. 82 is a load in which harmonics are generated, 125
Indicates a second AC current detector for detecting the AC current on the system side of the AC bus to which the load 82 and the self-excited converter are connected.

FIG. 8 shows the control circuit. Reference numeral 118 is configured similarly to the harmonic suppression signal generation circuit 117, and receives the output Iac of the second current detector as an input. The specific circuit of the harmonic suppression signal generation circuit 118 may be the same as that shown in FIGS. 4 to 6. The harmonic suppression signal created by the harmonic suppression signal creation circuit 118 is added in reverse phase to the modulated wave created by the control circuit output 115 of FIG. The generation circuit 119 compares the carrier signal (triangular wave) and generates a PWM pulse. In the self-exciting converter 42, an AC output voltage waveform is created by this PWM pulse, so an output voltage waveform in which the detected harmonics are suppressed is output.

As a result, the harmonics generated from the self-excited converter and the harmonics remaining in the AC system including the load can be suppressed.

[0041]

According to the present invention, since the control device of the self-exciting converter for DC power transmission has an active filter function, the generation of low-order harmonics is suppressed even with the same number of pulses of the self-exciting converter. The AC filter capacity of the conversion station can be reduced, and the cost of the conversion station can be reduced.

[Brief description of drawings]

FIG. 1 is a configuration diagram of a self-excited DC interconnection system including a self-excited converter according to an embodiment of the present invention.

FIG. 2 is a configuration diagram of a control circuit of a self-excited converter having an active filter function according to an embodiment of the present invention.

FIG. 3 is a configuration diagram showing details of a control circuit of a self-excited converter.

FIG. 4 is a configuration diagram showing an embodiment of a harmonic wave suppression signal generation circuit.

FIG. 5 is a configuration diagram showing another embodiment of a harmonic suppression signal generation circuit.

FIG. 6 is a configuration diagram showing still another embodiment of a harmonic wave suppression signal generation circuit.

FIG. 7 is a configuration diagram of a self-excited DC interconnection system including a self-excited converter according to another embodiment of the present invention.

FIG. 8 is a configuration diagram of a control circuit of a self-excited converter having an active filter function according to another embodiment of the present invention.

[Explanation of symbols]

1, 2 ... AC system, 21, 22 ... Impedance behind AC system, 31, 32 ... Transformer for connection (conversion), 4
1, 42 ... Voltage type self-exciting converter, 51, 52 ... DC capacitor, 60 ... DC transmission line impedance, 71, 72
... AC filter, 81 ... Load, 100 ... Operation command circuit,
110 ... Control circuit of converter 41, 111 ... AC current detector, 112 ... AC voltage detector, 113 ... Active / reactive power detector, 114 ... DC voltage detector, 120 ... Converter 42
Control circuit, 121 ... AC current detector, 122 ... AC voltage detector, 123 ... Active / reactive power detector, 124 ... DC voltage detector, 125 ... AC current detector, 115 ... Output voltage command value creating circuit, 116 ... Adder circuits 117 and 11
8 ... Harmonic suppression signal generation circuit, 119 ... PWM pulse generation circuit, 1150a ... First addition circuit, 1150b ... First operational amplifier circuit, 1151a ... Second addition circuit, 11
51b ... Second operational amplifier circuit, 1152 ... Signal selection circuit, 1153a ... Third adder circuit, 1153b ... Third operational amplifier circuit, 1154 ... Fourth adder circuit, 1155a
... fifth addition circuit, 1155b ... fifth arithmetic circuit, 11
56a ... Sixth addition circuit, 1156b ... Sixth operational amplifier circuit, 1157 ... Seventh addition circuit, 1158 ... Two-phase / three-phase conversion circuit, 1170 ... Band pass filter, 1171
... fundamental wave band-pass filter, 1173 ... constant multiplication circuit, 1174 ... first band-pass filter, 1175
Second bandpass filter, 117n ... Nth bandpass filter, 1176 ... Adder circuit.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Nagahashi Takahashi             2-2-1 Nakayama, Aoba-ku, Sendai City, Miyagi Prefecture             Tohoku Electric Power Co., Inc. (72) Inventor Hideto Kishibe             2-2-1 Nakayama, Aoba-ku, Sendai City, Miyagi Prefecture             Tohoku Electric Power Co., Inc. (72) Inventor Hiromichi Sato             2-2-1 Nakayama, Aoba-ku, Sendai City, Miyagi Prefecture             Tohoku Electric Power Co., Inc. F-term (reference) 5G066 CA04 EA03                 5H007 AA08 BB02 CA03 CC12 DA04                       DA05 DB02 DC02 DC05

Claims (7)

[Claims] 1. A controller of a self-excited converter for DC power transmission, wherein two self-excited converters for converting AC to DC and DC to AC are connected to each other via a DC circuit. The control device is composed of a vector control system that converts a three-phase alternating current into a two-phase direct current component of an effective component and an ineffective component, and from the current detected from each phase on the alternating current side of the self-exciting converter, a harmonic component to be reduced Of an active filter function for suppressing the generation of low-order harmonics by detecting for each phase, and is provided to correct the output voltage command value of the control device Control device. 2. A controller of a self-excited converter for DC power transmission, wherein two self-excited converters for converting alternating current to direct current and to convert direct current to alternating current are connected to each other via a direct current circuit. The control device is composed of a vector control system for converting a three-phase alternating current into a two-phase direct current component of an effective component and an ineffective component, and an alternating current detector for detecting the alternating current of the self-excited converter for each phase, A harmonic suppression signal creation circuit that detects the harmonic component that you want to reduce from the current detected from each phase for each phase, and multiplies the detected signal by gain for each phase,
Adding means for adding the output signal of the harmonic suppression signal generating circuit for each phase to the output voltage command value for each phase which is the prototype of the AC output voltage waveform of the self-exciting converter, and based on the added signal A control device for a self-excited converter for DC power transmission, comprising a pulse generation circuit for generating a pulse to control the self-excited converter. 3. A controller of a self-exciting converter for direct current transmission, wherein two self-exciting converters for converting alternating current to direct current and to convert direct current to alternating current are connected to each other via a direct current circuit. The control device is composed of a vector control system for converting a three-phase alternating current into a two-phase direct current component of an effective component and an ineffective component, and a first alternating current for detecting the alternating current of the self-excited converter for each phase. A second AC current detector that detects the current on the AC system side including the AC side current and load current of the detector and / or the self-exciting converter for each phase, and the harmonic component that you want to reduce from the current detected from each phase Is detected for each phase, a harmonic suppression signal creation circuit that multiplies the detected signal by gain for each phase, the output voltage command value for each phase that is the prototype of the AC output voltage waveform of the self-exciting converter Output signal of harmonic suppression signal generation circuit for each phase The addition means for adding, and the summed signal to create a pulse based on the self-commutated converter control apparatus of a DC power transmission self-commutated converter, characterized in that a pulse generating circuit for controlling. 4. The first alternating current detector according to claim 3, wherein one of the self-excited converters is provided with the first alternating current detector,
A controller for a self-excited converter for DC power transmission, characterized in that the second AC current detector is provided on the side of the other self-excited converter. 5. The direct current according to claim 2, 3 or 4, wherein in the harmonic detection of the harmonic suppression signal generating circuit, one harmonic order included in an alternating current is detected by a bandpass filter. Control device for self-exciting converter for power transmission. 6. The harmonic detection of the harmonic suppression signal generation circuit according to claim 2, 3 or 4, wherein some harmonic orders included in an alternating current are detected by some bandpass filters. A control device for a self-exciting converter for DC power transmission, characterized in that the added signals are added. 7. The DC according to claim 2, 3 or 4, wherein the harmonic detection of the harmonic suppression signal generating circuit detects all harmonic components except a fundamental wave included in an AC current. Control device for self-exciting converter for power transmission.